In industrial process measurement technology, process meters, so-called field meters, are used on site to generate measurement signals representative of analog or digital process variables, particularly in connection with the automation of chemical or other industrial processes. The process variables to be sensed may be, for instance, a mass flow rate, a filling or threshold level, a pressure, or a temperature. Such field meters are described, for example, in EP-A 984 248, U.S. Pat. Nos. 3,878,725, 4,308,754, 4,468,971, 4,574,328, 4,594,584, 4,617,607, 4,716,770, 4,850,213, 5,052,230, 5,131,279, 5,363,341, 5,796,011, 6,236,322, 6,397,683, or WO-A 00/36379.
To sense one or more process variables, the field meter has a suitable sensor, generally in the form of a physical-to-electrical transducer, which is mounted in a wall of a vessel holding or conveying a, e.g. liquid, powdery, vaporous, or gaseous, process medium, for instance in a pipe or a tank, and which serves to generate at least one measurement signal representative of the process variable being sensed, particularly an electric measurement signal.
The sensor is connected to suitable meter electronics, which serve in particular to process or evaluate the at least one measurement signal. Via a data transmission system coupled to the meter electronics, field meters of the kind described are linked together and to process control computers, where they send the measurement signal via (4- to 20-mA) current loops and/or via digital data buses, for example. For the data transmission systems, Fieldbus systems, particularly serial systems, such as PROFIBUS-PA, FOUNDATION FIELDBUS, and the corresponding communications protocols are used. By means of the process control computers, the transmitted measurement signals can be further processed and visualized as corresponding measurement results, e.g. on monitors, and/or converted to control signals for process control elements, such as solenoid valves, electric motors, etc.
To house the meter electronics, process meters of the kind described comprise an electronics case which, as proposed in U.S. Pat. No. 6,397,683 or WO-A 00/36379, for example, may be located at a distance from the field meter and be connected to the latter by a cord, or which, as also shown in EP-A 903 651 or EP-A 1 008 836, for example, is disposed directly at the field meter. Frequently, the electronics case, as shown in EP-A 984 248, U.S. Pat. Nos. 4,594,584, 4,716,770, or 6,352,000, for example, also serves to house some mechanical components of the sensor, such as diaphragm-, rod-, or sleeve-shaped bodies which deform or vibrate under mechanical action.
The advantage of a direct mechanical connection, particularly a rigid connection, between the electronics case and the sensor is that after installation of the sensor, virtually no further steps are necessary on site to fix the electronics case in position. However, any vibrations caused in the sensor or generated in the process and transmitted via the sensor, such as vibrations or pressure surges in a connected pipe, can be coupled into the electronics case nearly undamped. This coupling in vibrations, which is practically unavoidable particularly in the case of rigid connections, may, in turn, lead to vibrations with considerable amplitudes in the electronics case.
Acceleration forces or bending moments resulting from the case vibrations represent an increased mechanical load on the field meter as a whole and on the sensor supporting the electronics case in particular. During investigation of such case vibrations, acceleration forces of 10G (G=weight of the electronics case under test) were determined which caused deflections in excess of 50 μm.
It was also found, particularly in field meters with mechanical sensor components housed in the electronics case, that the case vibrations are superimposed as a spurious component on the useful component of the measurement signal or, in other words, are induced on the measurement signal as crosstalk.
If the case where vibrations have a resonant frequency which would lie approximately in the measuring range of the sensor or even in the range of a, e.g. operationally variable, frequency of the measurement signal, a separation of the useful component in the measurement signal from any spurious components may become virtually impossible. At any rate, since the meter electronics processing the measurement signal must be controllable over a comparatively wide signal level range and be highly selective while having a comparatively wide signal bandwidth, this separation of the useful component would require a considerable amount of additional circuitry, which would significantly increase the circuit complexity of the meter electronics and thus add to the manufacturing costs of the field meter.